Current bonding methods utilized for bonding LED devices, in particular high-power LED (“HP-LED”) devices, comprise approaches such as gold-tin eutectic direct die bonding and silver epoxy bonding. Conventional LED bonding often employs gold-tin eutectic bonding. About 1-2 μm of a 20% gold-tin eutectic layer is attached on the backside of a die, and it is used to bond an LED device on a substrate, for example, a ceramic or organic substrate with a metallization pad, or a lead frame. The process includes preheating the substrate to 300-310° C. in an ambient chamber with shielding gas, after which the LED die is picked and placed onto the heated substrate. The gold-tin eutectic layer will melt and wet on the bond pad on the substrate. Hence, the LED die is bonded onto the substrate by way of the gold-tin eutectic solder material.
However, along with LED power increases, the LED size has been enlarged from as small as 10 square mils to as large as 40 square mils. With this development, it is often a challenge to apply the eutectic bonding method to HP-LED bonding as it would produce large no-bonding portions or voids on the bonding interface, and the bonding outcome is unstable. As a result, large voids occur frequently in the bond area for gold-tin eutectic bonds. Bonding performance is further unstable due to the large die size and thin solder layer (of 1-2 μm). Bonding strength and shear force thus obtained from gold-tin eutectic die bonding frequently fail to meet the required bond specification for HP-LED. It is also expensive.
Another universally-applied conventional LED bonding method uses epoxy bonding. It involves dispensing epoxy with or without silver particles onto a substrate first, and then placing the LED die onto an epoxy dome formed from the dispensed epoxy. Post curing is then carried out in an oven to cure and solidify the bond with the adhered LED die. However, the method is not suitable for application to HP-LED bonding. With higher power, it has become increasingly necessary to get rid of the heat generated during use more efficiently, such that HP-LED packages have become more complex to adapt them for heat dissipation. The inherent low thermal conductivity of the epoxy compound generally cannot meet the requirement of HP-LEDs. It would result in high thermal resistance at the bonding interface and decrease the emission efficiency of the LED. Although heavy silver epoxy is being developed to improve the thermal conductivity thereof, it is difficult to use the technique with current technology to bond LEDs requiring high power, due to process issues such as formation of dispensing tails, difficulty in controlling the volume dispensed, and epoxy overflow on top of the LED if too much epoxy is dispensed. These process issues would result in the lack of bonding coverage, LED shorting, die tilting and other problems.
Due to the aforesaid shortcomings of conventional eutectic and epoxy die bonding, soft solder may be used instead for the packaging of high power electronic devices. Generally, solder wire is used by a wire feed dispenser, and it can be used to bond large dice (having die sizes of larger than 40 mils) with a bond line thickness of 1 mil to 3 mils. An additional spanking process is then employed to spread the dispensed soft solder uniformly. For power LEDs, solder volume control is important for meeting bonding requirements for thin bond line thicknesses so that there is no tilting and in order that the positioning can be precise. However, the solder dot placement position typically varies in the said solder wire dispensing process due to the non-uniform shape of the tip of the solder wire, wire deformation and random wetting on the pad. Clearly, the existing solder wire feeding and dispensing process has to be further improved for HP-LED bonding.
Solder balls have been widely applied to solder bumping for BGA devices, CSP devices, flip chip devices and other similar devices. Solder balls are picked up by vacuum suction for placing them onto the bumping pads of substrates or printed circuit boards (“PCBs”). The solder bumping apparatus includes suction holes for holding the solder balls, and a suction mechanism for generating negative pressure to perform vacuum suction. In BGA ball bumping processes, the pickup head has a bottom face formed with a large number of suction holes for vacuum-absorbing a plurality of solder balls from a container. The pickup head is moved into a solder ball container to absorb solder balls by applying negative pressure at its suction holes. A vibration mechanism is sometimes applied to remove extra balls around the pick tip of the solder bumping apparatus, or cause the solder balls to drop from the bottom face of the pickup head. Moreover, light is sometimes irradiated along at least one side of the bottom of the pickup head, so that the presence of a solder ball mounting face, or the absence thereof indicating a pickup error, can be determined.
The flux can be pre-printed or coated on the substrate or PCB, or by dipping the substrate or PCB in a flux solution. After the solder balls have been placed on the substrate or PCB pads with solder flux, a post reflow process is used to fuse and form the required solder bumps in a reflow oven or by utilizing a laser beam. In conventional solder ball processes, soldering flux is employed on the substrate or PCB pad to stick the solder balls to the respective surface during solder ball placement. The application of flux or other adhesive agents is undesirable since it involves an additional process step and has the disadvantage of low thermal conductivity.